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Thanks for all the answers The picture was taken by my colleague Roy Mangersnes, who had just been in Bolivia. Here is a link to a little slideshow from his trip:http://roywildphoto.blogspot.com/2009/11/slideshow-on-youtube.html
Opuntia fragilis, Escobaria vivipara... are the most cold hardy I can think of. If you can find a good northern clone you might have a chance with these, especially if you can keep them covered in snow during the coldest part of the winter.
All the South American cacti seem to top out at about the same hardiness and it isn't as hardy as you need. This one is found inside the tropics but at an altitude where you'd find it hard to walk across the road to take a picture. Then various species are found almost to the tip of Patagonia but at progressively lower altitude. The more southern forms to get exposed to more moisture with some of the Patagonian species more alpine than succulent.Opuntia fragilis, Escobaria vivipara, Escobaria missouriensis, and Pediocactus simpsonii, are the most cold hardy I can think of. If you can find a good northern clone you might have a chance with these, especially if you can keep them covered in snow during the coldest part of the winter.